This Small Business Innovation Research Phase II project will address the need for a trace trichloroethylene (TCE) vapor sensor with real-time measurement capabilities. TCE is a toxic volatile organic compound (VOC) used as an industrial solvent. TCE is a common soil contaminant at industrial toxic waste sites, and it migrates through the soil away from the original contamination site. TCE vapor intrusion into buildings from contaminated soil concentrates the TCE vapor indoors, where it poses a health risk to the occupants. Currently, TCE is monitored by capturing it with chemically active materials, and then analyzing those materials in a laboratory; the measurement interval is hours or days. A real-time monitor with a measurement interval of minutes would enable real-time mapping of the TCE concentration within a building, to locate vapor intrusion points of ingress and to monitor the quantity of TCE entering the building. The mapping distinguishes TCE entering the building from indoor sources of TCE. Entanglement Technologies proposes build a TCE vapor sensor based on the combination of cavity ring-down spectroscopy (CRDS) and diffusion time-of-flight (DiTOF) incorporating stationary phases. CRDS provides extremely sensitive detection while diffusion with stationary phase provides specificity. The objective for phase II is to build a commercial prototype analyzer to demonstrate TCE vapor detection in the presence of other volatile organic compounds (VOCs) and atmosphere (e.g. carbon dioxide and water vapor). The project will comprise constructing a self-contained CRDS/DiTOF prototype gas analyzer and demonstrating its performance in a field trial. The anticipated TCE sensitivity is better than 0.1 g/m3 (20 parts per trillion by volume (pptv)) in a measurement time of 10 minutes or less. The long term objective of this project is to develop a portable TCE vapor analyzer as a commercial product with a 0.1 g/m3 sensitivity. An additional objective is to adapt the same basic analyzer design resulting from this project to many different trace gases, including atmospheric and indoor-air pollutants, and combinations of trace gases. Such a family of analyzers will impact pollution research, control, and mitigation as much as CRDS carbon dioxide, methane, and water analyzers are currently impacting the study of greenhouse gases and climate change. CRDS/DiTOF technology will also be applied to biomedical science, industrial process monitoring, environmental remediation and explosives detection. For example, the diffusion-based selectivity will prove critical to the separation and quantification of the many hydrocarbon gas components in human breath useful for non-invasive diagnosis of disease. Similarly, CRDS/DiTOF can enable sensitive chemical analysis of liquids such as blood.